I don't know whether it was Heinlein or somebody else, but I remember reading the assertion that (and I'm paraphrasing) all laughter involves something painful at its core. I thought about that at the time and ever since, and have found not exception.

I'm not talking about intellectually appreciated wit or irony or a cute situation that makes you smile. I mean out-loud laughter. It was why I was interested to see what examples Lizzie gave.

I think laughter is how we exorcise negative emotions. You're afraid of something or someone, find a way to laugh at those fears; you're afraid of doing something stupid, find a way to laugh at the possibilities; you're afraid of mortality, make a joke; the pain you perceive someone may be in makes you feel ill, laugh at it. There's a line most decent people try not to publicly cross, and that's to not laugh at other people but at the pain or fear.

I think if we couldn't laugh, we'd all slit our wrists and humanity would die out or we'd turn into snooty types who think they're smarter than everyone else and humanity would die out (and there would be even MORE stupid political copy/paste flame threads before it did).